The invention relates to a driver circuit for driving voice and data signals from a switching center via a subscriber line to an end subscriber.
FIG. 1 shows a driver circuit according to the prior art. The driver circuit, which is located in the switching center, receives a voice signal via a signal input E1, which voice signal is emitted from a voice CODEC circuit to a voice signal driver. The voice signal driver can be activated and deactivated by the voice CODEC circuit. The driver circuit also contains a data CODEC circuit for receiving data to be transmitted, via an input E2 of the driver circuit. The data is emitted from the data CODEC circuit to a data driver circuit, which can be activated and deactivated via a control line by the data CODEC circuit.
The signal output from the voice signal driver circuit is followed by a passive low-pass filter, which emits the voice signals, after low-pass filtering, to a signal output A of the driver circuit. The data signal, amplified by the data driver circuit, likewise passes via a line to the signal output A of the driver circuit. The voice and data signal driven by the driver circuit is transmitted to a subscriber via a subscriber line whose line impedance is ZL, where it is split into a voice received signal and a data received signal by means of a splitter comprising a low-pass filter and a high-pass filter. The voice received signal is supplied to an analog telephone terminal for the subscriber while the received data is supplied to a modem. The data transmitted by the driver circuit from the switching center to the subscriber (downstream) is transmitted by means of the controllable data driver, the subscriber line and the high-pass filter to the subscriber""s digital data modem. In the opposite direction, the data transmitted from the subscriber""s modem is supplied (upstream) via the subscriber line to an external filter, from where it passes to the data CODEC circuit.
The driver circuit, which is located in the switching center, for the subscriber is in a quiescent operating mode (on-hook) when neither the analog telephone terminal nor the subscriber""s modem have been picked up. In the quiescent operating mode, both the voice signal driver in the driver circuit [lacuna] have high output impedance. When the analog telephone terminal is activated or picked up by the end subscriber, the series impedance, which is formed by the impedance of the subscriber line ZL and the impedance of the terminal connected in series, falls, and the direct current flowing from the driver circuit to the subscriber via the subscriber line rises. A pick-up identification circuit which is contained in the driver circuit and is connected via a switch S to the input of the passive low-pass filter in the quiescent operating mode identifies the current rise and emits a detection signal to the voice CODEC circuit. On receiving the pick-up identification signal from the pick-up identification circuit, the voice CODEC circuit changes to a working operating mode and, via a control line St1, activates the voice signal driver whose output impedance is low. Furthermore, on identifying the working operating mode, the voice CODEC circuit isolates the pick-up identification circuit from the passive low-pass filter by actuating the switch S, via the control line St2. The change in operating mode from the quiescent operating mode to the working operating mode results in the output impedance of the voice transmission driver circuit changing from a very high output impedance, which is several kiloohms, to a low output impedance of for example 600 ohms. The passive low-pass filter is provided to prevent this change in the output impedance of the voice signal driver from affecting the data transmission, suppressing the impedance change in the data frequency transmission band.
The passive low-pass filter has to have a very good filter characteristic so that its circuitry demands a relatively high level of complexity, and a circuitry implementation of the driver circuit occupies a relatively large surface area.
The object of the present invention is thus to provide a driver circuit for driving voice and data signals, which requires little circuitry complexity and in which the change between different operating modes has no effect on the transmission characteristics of the driver circuit.
According to the invention, this object is achieved by a driver circuit having the features specified in patent claim 1.
The invention provides a driver circuit for driving voice and data signals from a switching center via a subscriber line to a subscriber terminal having a controllable voice signal driver, which has a high output impedance in the quiescent operating mode (on-hook), and which, in a working operating mode (off-hook), drives an analog voice signal from the switching center via the subscriber line to the subscriber terminal with a low output impedance,
having a controllable data signal driver, which has a high output impedance in the quiescent operating mode and, in the working operating mode, drives an analog-modulated data signal in a data transmission frequency band from the switching center via the subscriber line to the subscriber terminal,
an identification circuit, which, in the quiescent operating mode, identifies a current rise in a current flowing via the subscriber line when the subscriber terminal is picked up, and produces a pick-up identification signal,
having a CODEC circuit, which, on receiving the pick-up identification signal from the identification circuit switches the voice signal driver from the quiescent operating mode to the working operating mode and disconnects the identification circuit from the subscriber line,
with the identification circuit having a frequency-dependent impedance, which, in the data-transmission frequency band, corresponds essentially to the output impedance of the voice signal driver in the working operating mode.
The data transmission frequency band is preferably between 26 kHz and 1.1 MHz.
The identification circuit which the driver circuit according to the invention contains preferably has a balanced circuit.
This balanced circuit preferably comprises two transistors, whose control connections are connected to one another.
The transistors are preferably bipolar transistors.
The bipolar transistors are preferably npn transistors.
The npn bipolar transistors preferably each have an emitter connection which is connected to a supply voltage via an associated resistor.
The identification circuit in the driver circuit according to the invention preferably also has a controllable MOSFET transistor which connects a complex impedance to the balanced circuit as a function of a control signal.
The following text describes preferred embodiments of the driver circuit according to the invention, with reference to the attached figures in order to explain features that are essential to the invention.